1. Field of the Invention
This invention is related to the filed of magnetic memories, magnetic sensors, and magnetic recording heads. More particularly, this invention is related to a domain wall free operative soft magnetic cell device.
2. Description of the Related Art
Giant Magneto Resistance (GMR) and its descendants have been commercialized in the read head sensor of a magnetic recording head and in various other magnetic sensors. Companies are currently in the development stage to commercialize magnetic random access memory (MRAM) based on Tunneling Magneto Resistance (TMR) effects. Current issues in industry in magnetism include (1) higher speed (magnetization dynamics), (2) lower size, (3) patterned media, (4) concern over the super paramagnetic limit and others. Most industrial needs require micro structurally patterned thin-film magnetic elements.
Any magnetic sample with boundaries is difficult to magnetize completely right up to the boundaries. In particular, for technologically interesting thin film micro structured elements, closure domains form in the remnant state that partially demagnetize the element. As is known in the art, the term “domain” generally refers to a part or portion of a magnetic material in which magnetization is generally uniform and which behaves as a single magnetic entity. Such remnant states are undesirable for several reasons: they decrease remnant magnetization (for instance, lowering the maximum signal in a memory cell), they complicate magnetic structure and tend to complicate magnetic behavior, making study more difficult, and, in particular, they adversely effect the understanding and practice of high-speed magnetization processes due to their non-uniformity. Increased uniformity of magnetization, combined with reproducibility, is often desired in both research and device operation.
The shape of an ellipsoid, by its mathematical nature, may mimic magneto static continuity, suppress closure domains, and lead to uniform magnetization. For extremely small samples, the exchange interaction dominates magnetostatics, and is theorized to be able to lead to uniform magnetization (and, thus, suppression of closure domains). These are well known ideas in magnetism, available since the 1950s. Recent review books covering these ideas are (1) A. Aharoni, Introduction to the theory of ferromagnetizm, 2nd Edition, Oxford University Press, Oxford (2000). (2) A. Hubert and R. Schafer, Magnetic domains (The analysis of magnetic microstructures), Springer-Verlag, Berlin Heidelberg (1998), each of which is incorporated herein by reference in its entirety.
But real thin film ellipses are not ‘exactly’ ellipsoids and have been known to have closure domains, such that a uniform magnetization leading to substantially 1 single magnetic domain haven't been achieved.
Memory cells, spin valve GMR elements, and magnetic sensors currently                (a) Ignore end domains; but end domains reduce GMR signal and increase variability from cell to cell (e.g. switching fields have too much variation amongst cells that are supposed to be identical).        (b) Use a hard bias for GMR read head element to sweep domains out of the element; but a hard bias lowers sensitivity.        (c) Play with cell shape to reduce end domains and/or their effects; but playing with sample shape is difficult and time-consuming for fabrication. Moreover end domains are rarely eliminated.        
Various methods to handle end domains exist:                (a) Pin cell ends so that switching is less variable (U.S. Pat. No. 5,748,524, ‘MRAM with pinned ends’, Chen et al., which is incorporated herein by reference in its entirety); here some TMR or GMR signal is lost due to the pinned ends not contributing.        (b) Edge closer structure in laminates in recording head yoke (U.S. Pat. No. 5,331,728, ‘Method of fabricating magnetic thin film structures with edge closure layers’, Argyle et al., which is incorporated herein by reference in its entirety); which is difficult and time-consuming to produce.        (c) Induce a single domain magnetic state by providing a vertical flux closure path (patent no. EP 0 875 901 A2, which is incorporated herein by reference in its entirety) This is done by adding a magnetic region at the side edges of a first and a second magnetic layer that make up a magneto-resistive memory cell. A closed magnetic circuit is formed vertically to the plane of the cell and involves both the first and second magnetic layer to provide the flux closure path.        (d) Induce a single domain magnetic state in a 3D ellipsoidal magnetic cell. In WO 02/35559 A2, which is incorporated herein by reference in its entirety, a vortex-free single domain state is induced by adding layers with an area less than the area of the original layer. The smaller layers act to provide a reduced and more uniform demagnetizing field for the total bit, thereby inhibiting vortex formation and improving the operating characteristics of the magneto-electronic element. The most preferred configuration is a 3D ellipsoidal element that theoretically has uniform demagnetizing field for the total bit element.        (e) In U.S. patent application US2002/0055190, which is incorporated herein by reference in its entirety, (‘Magnetic memory with structures that prevent disruptions to magnetization in sense layer’, Thomas C. Anthony), a keeper structure providing a flux closure path to direct demagnetization fields away from the sense layer is disclosed. However, the magnetization in the keeper and the cell are always perpendicular to each other and no single magnetic domain state is induced in the cell.        
In [Z. Qian et al., IEEE Transactions on Magnetics, vol. 39, NO.5, SEPTEMBER 2003, p3322, which is incorporated herein by reference in its entirety], a similar method adds a third magnetic layer to a magneto-resistive cell with a first and a second (sense) layer. The third layer has a predetermined magnetization anti-parallel to the first magnetic layer to provide a vertical flux closure path for the first magnetic layer, This method reduces the fringing fields from the first layer, thereby improving the operating characteristics of the second layer.
Similarly in document JP06036275, which is incorporated herein by reference in its entirety, a thin film is added under a vertical magnetic recording medium, as well as a keeper structure at the outer and inner peripheral end. The soft layer and the keepers concentrate the magnetic flux in the recording head.
In U.S. patent 2001030886, which is incorporated herein by reference in its entirety, insulating regions with large relative permeability are added around the memory cell, the bit lines and the word lines. This is to improve the coupling of the magnetic fields generated by the bit lines and/or word lines to the magneto-resistive memory cell as to reduce the currents necessary to change the magnetization state of the memory cell.
However those last three inventions do not imply single domain behavior of the magnetic cell.